The present invention relates to surgical implant devices, especially implant devices utilized in procedures where rigid internal fixation of bone portions is desired.
Various orthopedic pins and fasteners have been devised to aid in properly fixating bones or bone portions so as to promote as much as possible the primary healing of fractured bones to their original condition. The most common devices used to attain fixation of portions of fractured bones include and rely upon screw fasteners which must be entered and advanced into the bone structures worked upon. Newer devices which have not yet gained wide acceptance provide elongate fasteners which are of relatively narrow configuration when inserted into bone being worked upon and which operate to expand laterally or radially after being inserted to attain fixation in the bone. These devices are commonly classified or called "expanding nail devices".
Both the above noted screw type devices and expanding nail type devices are deficient in several important respects. To comprehend the nature of the deficiencies of these prior art devices, the structural characteristics of living bone must be considered.
Living bones are formed of cancellous material within a surrounding cortex. The compressive strength of compact cortical bone is 10,000 to 35,000 pounds per square inch while shear strength is considerably less. The cortex encases the much softer spongly cancellous bone which yields under very small pressures.
In the case of common screw type devices, such as "sliding bone screws" used in the hip, the devices are placed in bone being worked upon by drilling holes though the cortex of the trochanter region and through the cancellous material thereof across the area of fracture and into the cancellous material of the femoral head of the bone. The only forces compressing the proximal and distal portions or opposite portions of the fractured bone are provided by the threads of the screws gripping into the cancellous bone of the femoral head and the ends of the screws (and related plates) acting against the cortex of the femur. Since the cancellous bone of the head is a yielding, spongy material, very little compressive or fixative force is provided by a conventional screw-type device or "sliding bone screw".
The expanding nail type devices were devised to provide fixation devices which more effectively fixate the portions of fractured bones to promote proper healing. One such device is described in my U.S. Pat. No. 4,409,974. The device which is the subject matter of my above noted patent is an implant device characterized by a tubular member having a frustoconical head and a plurality of traingular shaped arms. That implant device is inserted into a bore in the bone worked upon, across the fracture in said bone, with the arms in a radially inwardly folded or collapsed position. Once inserted, tension is exerted on trailing lines emanating from an opening in the tubular member remote from the head thereon. By pulling the lines, the arms are pivoted or rotated from said folded position to radially outwardly disposed anchoring positions within the cancellous material of the bone. That implant device can be collapsed for removal or for repositioning by folding the arms back or radially inwardly. This is accomplished by pulling a generally cylindrical mesh sleeve disposed about the tubular member and connected with the arms by filament, longitudinally of the tubular member.
One distinct advantage of my above noted patented device over prior art screw type and expanding nail type devices resides in the fact that the arms, when deployed or disposed radially outwardly, provide fixation through compressive forces on the bone to reduce fractures and to fixate bone portions, whereas common screw type devices rely on "shear cylinder" forces applied directly onto and through the bone. Also, the nature of my above noted patented device is such that by increasing the cross-section of the arms, the surface area of the arms acting in the material of the bone is increased and notably greater forces can be applied onto and through the bone worked upon, without weakening it. In the case of screw type devices, to generate greater forces the screws must be made greater in diameter. Such increases in the diameter of screws requires that the diameter of the bores or openings established in the bone to receive the screws must be increased accordingly, intrinsically weakening the bone, especially upon removal of the devices.
My above noted patented device had many design advantages over prior art expanding nail type devices. As many of those design advantages have been incorporated into my new device which is the subject matter of the present invention, comparison of the prior art and my present invention will be set forth in the following description of a preferred embodiment of my present invention.